Sight glasses used for observing the presence of fluid within a conduit are well known. Typically, such sight glasses are comprised of a glass tube supported in a fitting with its bore open at its respective ends to the fluid supply and exhaust.
Another example of such a sight glass is illustrated in U.S. Pat. No. 2,744,487, issued May 8, 1956, the sight glass of this patent including spaced observation windows mounted in a fitting having an axial bore.
Such sight glasses have many uses, including their use in conjunction with fire extinguishing sprinkler systems, and, in particular as a means for observing water flow through a test line of such a system.
Typically, the test line is connected as a branch line to the upstream end of a feeder main of the sprinkler system, the feeder main carrying the individual sprinkler heads.
For the purpose of determining operativeness of the sprinkler system, and in particular of an alarm switch associated with the feeder main, the test line is provided with a test valve, which in turn is connected with a sight glass, the sight glass in turn being connected with a metering orifice exhausting to a drain line. The metering orifice is so dimensioned that, on opening of the test valve, a flow of water occurs in the test line simulating in volume with the flow of water through a single one of the smallest individual sprinkler heads when in the activated condition.
Thus, on opening of the test valve, a water flow occurs in the main feed simulating that which occurs on activation of a single sprinkler head, and, the water flow switch located upstream of the test line assumes an actuated position.
The sight glass is provided in order that water flow in the test line may be visually observed.
In the absence of such water flow, a condition which could occur as a consequence of the test valve being defective, or, by reason of blockage of the metering orifice or of the drain line, no indication can be obtained as to whether the water flow switch is operative or defective, or, whether the water flow switch is set for actuation at exactly the water flow produced by a single sprinkler head.
On opening of the test valve, the sight glass only momentarily provides an indication of the commencement of the water flow, for it is immediately filled with a solid core of water. Such a solid core of water is not readily distinguishable from an empty sight glass, particularly in poor light, or, if the sight glass is dirty.